This invention relates to current regulators and particularly to a precision regulator for controlling and monitoring current to a cathode of an electron tube such as a cathode-ray tube during testing of the tube.
After a cathode-ray tube, such as a color picture tube, has been fabricated, it is necessary to test the tube to ensure that it will operate within desired tolerances. In order to attain uniformity in testing, it is important that the cathode current being supplied to the tube is accurately controlled. To date, several different types of regulators have been tried to perform this regulation. These different types have employed components such as bipolar transistors, vacuum tubes and field effect transistors. Of these components, recent developments in improving field effect transistors have made these components a preferred choice. However, there are drawbacks in using field effect transistors. These drawbacks include drift or change of current due to change in the transistor characteristics caused by temperature sensitivity and the risk of transistor damage caused by high voltage transients.
In addition to providing regulated current, it is also desirable to be able to measure the actual cathode voltage when operating at the regulated current level without actually changing that value due to the metering employed. When the circuit is regulating to a low value of current at a high input voltage, the regulator represents a high impedance. FIG. 1 illustrates the impedance of a regulator versus current and voltage. Normal methods of voltage metering require a resistive shunt which would alter the test results. Further, the shunt load also represents a non-regulated current path which would degrade regulator performance.
The present invention provides a new current regulator circuit which has better regulation than the previous concepts, is more stable and less sensitive to thermal drift, has provision for cathode voltage measurement, and takes advantage of the properties of a field effect transistor which allow operation in the depletion and/or enhancement modes.